A WDM network often employs ring-type or mesh-type network topology. In the WDM network of any type, a plurality of node apparatuses are interconnected via a route. The route is a communication path that interconnects the node apparatuses, and includes a pair of optical fibers for transmission and reception. It should be noted that the node apparatus is called an optical node apparatus when node apparatus directly processes and outputs an optical signal without converting the input optical signal into an electric signal. In the specification, a node apparatus that converts an input optical signal into an electric signal to process the optical signal or the electric signal, and the optical node apparatus are generically named an optical transmission apparatus. An optical node apparatus that interconnects a plurality of ring-type networks, or an optical node apparatus that constitutes a mesh-type network is called an optical hub node apparatus that includes three or more routes to other optical transmission apparatuses and switches the plurality of routes.
The optical hub node apparatus incorporates a plurality of route units as interfaces for connection with other optical transmission apparatuses. In the optical hub node apparatus, all the route units are connected to all the other route units via the optical fibers for transmission and reception. In other words, in the optical hub node apparatus, the plurality of route units for transmission and reception are completely interconnected in a meshed manner. The route unit separates an optical signal of an arbitrary channel (wavelength) from optical signals input through an uplink (reception) optical fiber from the other optical transmission apparatuses to output the optical signal to an arbitrary route unit. The route unit couples together optical signals of one or more channels input from the other route units to output the coupled optical signals to the other optical transmission apparatuses through a downlink (transmission) optical fiber.
In the aforementioned WDM network, optical transmission apparatuses or routes may be added as demand thereof increases. However, the number of route units incorporated in the optical hub node apparatus is predetermined, and hence the number of optical transmission apparatuses (number of routes, degree, or number of edges) allowed to be connected to the optical hub node apparatus cannot be increased unlimitedly.
The following are related arts to the invention.    [Patent document 1] Japanese Patent Laid-Open Publication No. JP 2006-191212    [Patent document 2] Japanese Patent Laid-Open Publication No. JP 2004-527945    [Patent document 3] Japanese Patent Laid-Open Publication No. JP 11-331224